The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
With regard to ease of storage, natural gas is frequently transported by pipeline from the source of supply to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when the supply exceeds demand. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to -240.degree. F. to -260.degree. F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen or combinations of the preceding refrigerants (ex. mixed refrigerant systems). A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e, the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.
In any liquefaction process producing a pressurized LNG-bearing stream, the presence of nitrogen and/or other low boiling point inorganic components such as helium is problematic because of the solubility of these components in pressurized LNG. Further, elevated concentrations of these components in the open methane cycle can increase refrigeration requirements and result in various operational problems. The removal of such components is required at some location in the process. One methodology for such removal has been to flash the pressurized LNG-bearing stream and employ the resulting flash gas stream(s) as fuel gas for drivers (ex. turbines) for refrigerant compressors employed in the liquefaction processes and/or electrical generators. However, the development of more environmentally-friendly turbines (ex. low NOX capability) has been accompanied by more stringent fuel gas requirements, most notably an increase in the minimal BTU content of the fuel gas. Therefore, conventional schemes for removing nitrogen from a liquefaction process via a fuel gas stream may no longer be practical when the BTU content of the flash gas stream(s) is too low for desired turbine operation. Further, fluctuations in fuel gas quality attributed to process upsets may render such conventional methodologies impractical. When there is little demand for fuel gas (ex. electric drivers are employed), the need to remove nitrogen from the liquefaction process in a manner which produces at least one low BTU nitrogen-rich gas stream which may be vented, used as a nitrogen source or used as a purge gas and at least one high BTU methane-rich gas stream which can be easily recycled to the liquefaction process becomes even more desirable.